Real-Time Tunable Gas Sensing Platform Based on SnO2 Nanoparticles Activated by Blue Micro-Light-Emitting Diodes

Highlights Blue micro-light-emitting diodes (μLED)-integrated gas sensors were fabricated as monolithic structure by directly loading sensing materials onto the μLED. SnO2 nanoparticles are activated by blue μLED and exhibit outstanding sensitivity to NO2 at μ-Watt power levels. Noble metal (Au, Pd, Pt)-decorated SnO2 showed the tunable gas selectivity for 4 target gases under blue light illumination. Supplementary Information The online version contains supplementary material available at 10.1007/s40820-024-01486-2.


Introduction
The gradual increase of the pandemic has enlightened the public about the significance of real-time detection of hazardous substances in daily life.Specifically, the detection of gaseous analytes has become more essential due to the direct association with human health.The growing demand for harmful gas detection has triggered the development of various types of gas sensors, including electrochemical sensors, surface acoustic wave sensors, optical sensors, and chemoresistive sensors [1,2].Among these, chemoresistive gas sensors have gained tremendous attention owing to their simple structure, small size, and facile operation.The fundamental operation principle is based on measurement of the change in electrical current of the sensing materials upon exposure to the target gas.Metal oxides are the most widely utilized materials in chemoresistive gas sensors [3].However, at room temperature, metal oxides-based chemoresistive sensors suffer from low response and irreversible recovery [4].To overcome these drawbacks, external heating systems were adopted to elevate the operation temperature, including back heater and microelectromechanical (MEMS) systems [5,6].Despite improved gas sensing properties through thermal activation, high power consumption and thermal degradation of sensing materials remain critical issues, which are unfavorable for miniaturization and stability.Thus, extensive research on room temperature operable gas sensors for high reliability and stability has been extensively conducted.
Recently, light activation using light-emitting diode (LED) has been extensively utilized for room temperature operation with reduced power consumption [7,8].Light irradiation on the sensing material generates the photo-carriers and forms the reactive photo-activated oxygen, which is reactive with target gases [9,10].Ultraviolet (UV) light has been widely utilized in the activation of metal oxides due to the large bandgap of metal oxides [11].However, UV light is known to harm the human body and cause skinrelated diseases such as skin cancer, premature aging, and burns [12,13].To address this drawback, in our previous work, we fabricated SnO 2 nanoparticles (NPs) gas sensor activated by visible light illumination, which does not harm human skin [14].The defect states in SnO 2 NPs enabled the photoexcitation under visible light, resulting in room temperature NO 2 detection with high response and selectivity.Despite these promising features, light-activated gas sensors based on SnO 2 NPs still have limitations that need to be addressed.The use of commercial LEDs requires high power consumption compared to MEMS heaters, and the distance between the LED and the sensing material results in energy loss due to light diffusion.Furthermore, most research on light-activated gas sensors has focused on a single target gas detection, especially NO 2 .Detecting a variety of hazardous gases, such as NO 2 , NH 3 , H 2 , and VOCs, is essential for protecting human health.However, since NH 3 , H 2 , and VOCs are less reactive than NO 2 at room temperature, it is challenging to detect these gases using visible light-activated techniques at room temperature [4,15].
Herein, we introduce a fully hardware-implemented blue μLED-integrated gas sensor array based on noble metal decorated SnO 2 NPs with controllable gas selectivity.By using μLED instead of commercial LED, power consumption is significantly reduced from milli-watt to micro-watt.Furthermore, the μLED-integrated gas sensor reduces the distance between the sensing material and the light source, which can substantially be efficient to power consumption and increase energy efficiency.The compact configuration allows for application on various platforms, including wearable devices, medical devices, and smart home systems, expanding the potential applications of μLED-integrated gas sensors.SnO 2 NPs, previously studied as sensing materials, were deposited on μLED-integrated gas sensor [14].Under the blue light, the 10 nm-sized SnO 2 NPs exhibited excellent sensing performance, including high response, fast response and recovery, high reliability, and low detection limit.Finite-difference time-domain (FDTD) simulation was conducted to reveal the mechanism of the characteristics of gas response.Furthermore, noble metals (Au, Pd, and Pt) were decorated on SnO 2 NPs to detect gases such as NH 3 , H 2 , and C 2 H 5 OH with fabrication of μLED gas sensor array.While several studies have been published that apply noble metal decoration to modulate gas selectivity, these studies are limited to high temperature environments [16,17].By applying noble metal decoration on SnO 2 NPs, which has superior light activation properties, the catalytic effect of noble metals has enabled selective gas detection under blue light illumination.Moreover, the realtime wireless gas monitoring was demonstrated with fully μLED-hardware-integrated gas sensor array by utilizing a microcontroller unit (MCU) and customized printed circuit board (PCB).This study will establish a guideline and pave the new avenue for advancement in electronic nose (e-nose) technologies.

Fabrication of μLED Platform
μLED gas sensor platform was fabricated by following procedures.Conventional LED on sapphire wafer with 8 pairs of superlattices and 4 pairs of multiple quantum wells was prepared into chip scale.A 2 × 2 array of square patterns was formed on the LED film via photolithography followed by inductively coupled plasma etching to vertically etch the LED film to form a mesa structure.Then, the current spreading layer composed of 10 nm Ni and 10 nm Au was deposited on the p-type GaN region by photolithography and e-beam evaporation.After the lift-off process of Ni/Au metal stack, the sample was annealed at 500 °C in ambient air by a rapid thermal annealing (RTA) system to form an ohmic contact between the p-type GaN layer and current spreading layer by oxidizing the Ni layer [18][19][20].The ultrathin Ni/Au layer would turn into a transparent layer after the thermal annealing process.After that, n-type contact metal, composed of 20 nm Cr for adhesion and 200 nm Au for current flow, was formed on the n-type GaN layer by photolithography and e-beam evaporation.To connect the current spreading layer to the p-type contact metal that would be formed later, a small region of the Cr/Au metal layer was also formed on the current spreading layer during the process.For electrical insulation, 700 nm SiO 2 was deposited on the sample by plasma-enhanced chemical vapor deposition (PECVD).The contact region of the metal layer was opened by photolithography and reactive ion etching (RIE).On top of that, the lift-off pattern was formed by photolithography, and p-type contact metal also composed of 20 nm Cr and 200 nm Au was deposited by e-beam evaporation.An additional insulation layer of 700 nm thick SiO 2 was deposited by PECVD followed by RIE etching to expose the contact region.Finally, 20 nm Cr and 200 nm Au electrodes were deposited on the LED via photolithography and e-beam evaporation.The overall fabrication process of μLED platform is presented in Fig. S1.under constant stirring for 3 h at 100 °C.The centrifugation was conducted to remove the residues after the decoration process and doping process.The products were collected after completely drying the powders.

Gas Sensing Measurements
For gas sensing measurements, a slurry was prepared by mixing SnO 2 NPs with a terpineol-based ink.The ink was then uniformly coated on Cr/Au electrodes and dried on a hot plate.This process was repeated three times to fully cover the sensing area of Cr/Au electrodes.The RTA was conducted under N 2 atmosphere at 350 °C for 10 min to improve the crystallinity of SnO 2 NPs and evaporate the solvent.The prepared sensor was placed inside a quartz tube, and voltage was applied to both μLED electrodes and sensing electrodes.The resistance was recorded by I-V source meter (Keithley 2625) under an applied voltage of 0.5 V. Dry air was continuously supplied inside the quartz tube with the assistance of mass flow controllers (MFC).The target gases, prepared in air balance, were inserted into the quartz tube at specific concentrations.
The gas concentration was controlled by adjusting the flow ratio of air and the target gases, maintaining a total flow rate of 1000 sccm.Humid air was generated by passing dry air through a water bubbler.To control the humidity level, the generated humid air was mixed with additional dry air using MFC.Real-time gas monitoring of the μLED gas sensor array was conducted in a desiccator to control gas environment.

Characterizations
The electrical properties of the fabricated μLEDs were characterized by Keithley 4200A.For optical measurement, a source meter (Keithley 2602B) was used for supplying the current to the sample, while a Si p-i-n photodiode (Hamamatsu Photonics S2281-04) and a fiber-optic spectrometer (AvaSpec-2048) were utilized for analysis.The morphology of SnO 2 NPs was characterized by field emission scanning electron microscope (FE-SEM, Hitachi, SU70) and transmission electron microscope (TEM, JEM 2100F).X-ray diffraction (XRD) analysis was conducted by D8-advance, BRUKER MILLER Co. X-ray photoelectron spectroscopy (XPS) spectra were obtained by AXIS-HIS, KRATOS with Al Kα (1486.6 eV) X-ray source at 25.3 W. The UV-Vis absorbance spectra were measured by UV-Vis spectroscopy (770, JASCO), and photoluminescence (PL) spectrum was recorded by a micro-PL system (Dongwoo Optron) with a 325 nm He-Cd laser.The laser spot size of the micro-PL system used in this study was 3 µm.The investigation on substrate temperature change under LED illumination was conducted by IR camera (AX5 series, FLIR).

Finite-Difference Time-Domain Simulation
Electromagnetic simulations proceed with a finite-difference time-domain (FDTD) program (Lumerical Solutions).
Refractive indices of SnO 2 and SiO 2 are adopted from Mohamed [21] and Palik [22].36 layers of 10-nm-sized SnO 2 NPs with close-packed FCC structure are arranged in over SiO 2 planar thin film in air.Periodic boundary conditions were adopted for the x and y dimensions, and perfectly matched layer conditions were adopted for the z dimension.
A plane-wave source is adopted, and it propagates from − z to + z (forward) direction (SiO 2 to air).An unpolarized wave is adopted by averaging s and p-polarized waves.A 0.5-nmsized cubic size cell was used for meshing.

Characterization of μLED-Integrated Gas Sensor Array
Figure 1a presents the simplified structure of the μLED platform for gas sensing measurement.The device consists of 4 individual μLEDs to fabricate gas sensor array for discrimination of different target gases.The optical image of μLED gas sensor platform with dimensions of 20 × 20 μm 2 is shown in Fig. S2.Each μLED has p-and n-contact electrodes for operation and sensor electrodes for gas detection with a SiO 2 passivation layer between them.The light passes through the SiO 2 layer and photo-activates the sensing material on the device, which is located between the two sensing electrodes, as shown in Fig. S3 [23,24].Figure S4 shows the transmittance of SiO 2 layer, indicating that the SiO 2 passivation layer has little effect on the light emitted by the blue μLED.Figure 1b illustrates the enlarged structure image, wherein SnO 2 NPs were directly loaded onto the μLEDs platform, establishing electrical connections with the sensor electrodes.The blue light emitted from the μLEDs activates SnO 2 NPs, while the target gas is introduced to the device through a quartz tube and reacts with SnO 2 NPs.The resistance signal is read from the sensor electrodes as target gases react with SnO 2 NPs. Figure 1c shows a photograph of a 2 × 2 array of μLED-integrated gas sensor platform, with the device of 1 × 1 cm 2 .The sensing material was coated using a micro-needle, as shown in Fig. S5a.The ink droplet was prepared by dispersing the sensing material in the terpineolbased solvent.The microneedle was dipped into the prepared ink, forming a small droplet.The ink droplet was cast on the sensing area, as displayed in the optical microscope image in Fig. S5b.The sensing material covered the LED region and successfully filled the space between the sensor electrodes, electrically connecting the two electrodes.The uniform coating of the sensing material on the μLED was confirmed by SEM image in Fig. S6.Using this method, the sensing materials are deposited in a cost-effective and simple manner without the need for additional lithography steps.Figure 1d shows emission images of the μLED under different applied voltages: 2.8, 3.1, 3.4, and 3.7 V.The electrical properties of μLED are presented in Fig. S7a, where ohmic contact formation was confirmed from the current-voltage (I-V) measurement, with an estimated turn-on voltage of 3.4 V.The effect of the SiO 2 insulating layer was confirmed by the extremely low reverse leakage current of 1.2 × 10 -10 A at − 5 V as presented in the inset of Fig. S7a. Figure S7b depicts the emission intensity of μLED under different injection currents, where the emission intensity demonstrates an apparent linear increase, supporting the formation of ohmic contact at the interface between the current spreading layer and p-type GaN.The device emitted a sharp blue light characterized by a peak wavelength of 453 nm and a full width at half maximum (FWHM) value of 20.95 nm at an operating power of 63.2 µW, as shown in Fig. 1e.External quantum efficiency (EQE) of the 20 × 20 μm 2 sized μLED was measured to be 2.91%, as shown in Fig. S8.The EQE of μLED was measured to be lower than the EQE of commercial LEDs due to the size effect of the μLEDs [25][26][27].However, the low EQE has a marginal effect on the sensing performance, as the sensing material is located directly above the μLED.The distance between the light source and sensing material is less than 1.5 μm, minimizing the energy loss of light emitted from the μLED.As depicted in Fig. S9, the μLED exhibited a blue shift of the emission wavelength as the injection current increased due to the quantum confined Stark effect (QCSE) [28,29].However, our device operates at a low current level, minimizing the effect of QCSE on emission performance.Overall, the μLED platform was successfully fabricated and emitted sharp blue light with extremely low power consumption.For gas sensing material, SnO 2 NPs were prepared by hydrothermal method.SnO 2 is a direct bandgap material, which can effectively absorb photon emitted from μLED [30].Furthermore, SnO 2 has dual valency with an Sn oxidation states of Sn 2+ and Sn 4+ [31].This dual valency of Sn facilitates the formation of Sn interstitials and oxygen vacancies with low formation energy [32].These interstitial and vacancy sites generate the trap sites between conduction band and valance band, enabling SnO 2 to absorb light in visible range, which is lower energy than the bandgap of SnO 2 [33].Figure 1f showed SnO 2 NPs with an average particle size of 10 nm.High-resolution images were obtained using TEM to investigate the crystallinity of the SnO 2 as shown in Fig. S10.The crystal structure of each material was confirmed with diffraction patterns, which are presented in the inset image.The lattice fringe was observed to be 0.33 nm, corresponding to the (110) plane of SnO 2 [34,35].The XRD spectra of SnO 2 NPs exhibited clear peaks, which indicates the formation of rutile SnO 2 validated by JCPDS No. 41-1445, as exhibited in Fig. S11 [31,36].The chemical state of SnO 2 was studied by XPS analysis, as depicted in Fig. S12.The Sn 3d 3/2 and Sn 3d 5/2 peaks of SnO 2 appeared at 495.05 and 486.65 eV, respectively.The binding energy difference between Sn 3d 3/2 and Sn 3d 5/2 peak was measured to be 8.4 eV, which is in good agreement with the chemical state of Sn 4+ [37,38].The O 1s spectra of SnO 2 NPs were subjected to deconvolution, resulting in the identification of three distinct peaks corresponding to different oxygen states.The O ad peak, which indicated the oxygen adsorbed on the surface of SnO 2 , appeared at 531.75 eV.The O V peak, representing oxygen vacancy in SnO 2 , appeared at 530.75 eV, while the O L peak, corresponding to oxygen at regular lattice sites of SnO 2 , appeared at 530.25 eV.
UV-vis spectroscopy was employed to evaluate the optical characteristics of SnO 2 , as shown in Fig. 1g.The bandgap was determined by analyzing the absorbance spectrum with a Tauc plot, as shown in the inset image.The bandgap of SnO 2 was determined to be 3.6 eV, which is a commonly observed value for SnO 2 [39,40].Photoluminescence (PL) analysis of SnO 2 NPs was conducted, as shown in Fig. 1h.The spectrum exhibited a wide range of energy states, suggesting that the presence of tin interstitials and oxygen vacancies led to the formation of intermediate energy levels [41].Due to the presence of vacancy states, SnO 2 NPs could be activated by blue light despite its high bandgap energy.The peak emission wavelength of SnO 2 NPs was measured to be approximately 450 nm, which is comparable to the emission wavelength of our μLEDs.The optical analyses confirmed that SnO 2 NPs can be activated within the visible range, particularly by blue light.

Gas Sensing Performance of SnO 2 NPs
I-V characteristic curves of SnO 2 NPs under different light intensities were measured to investigate the photocurrent of SnO 2 NPs, as shown in Fig. S13.The operating power was calculated using Eq. ( 1): As the power consumption of μLED increased, the photocurrent of SnO 2 NPs gradually increased.Gas sensing measurements of SnO 2 were conducted for 5 ppm of NO 2 under different μLED operating powers, as shown in Fig. 2a.In dark condition, the baseline resistance of SnO 2 was significantly high, making it difficult to read by sourcemeter, and the response to NO 2 was discernible.When SnO 2 is exposed to blue light, photocarriers are generated, which leads to a substantial decrease in the base resistance of SnO 2 .As the power consumption of μLED increased, the baseline resistance decreased, as shown in Fig. S14.When NO 2 is introduced, SnO 2 NPs showed the response with an increase in resistance.After 500 s of NO 2 exposure, air was introduced, and the resistance of SnO 2 NPs began to recover.The mechanism of light-activation in gas sensors is detailed in Note S1.The relationship between response and power consumption is plotted in Fig. 2b, showing a volcano-shaped plot.The response of NO 2 , denoted as R o , was defined using Eq. ( 2), which represents the response to oxidizing gas as the ratio of the resistance variation in the air and target gas atmospheres.
Here, R g represents the resistance in the gas atmosphere, while R a represents the resistance in the air environment.At low light intensity, the response to NO 2 increases as the intensity of the light increases.However, the response (1) decreases as the light intensity surpasses the optimal point, which is determined where the response reaches the maximum value.At the optimal point, the response to 5 ppm of NO 2 was 6928 with a power consumption of 63.2 μW.The response time is defined as the duration required for the resistance to reach 90% of the fluctuation after being exposed to the target gas.As shown in Fig. S15a, the recovery time decreases as the light intensity increases due to the photogenerated holes engaged in NO 2 desorption.In contrast, the response time increases as the light intensity increases, as shown in Fig. S15b.Higher light intensity increases the number of active sites due to the desorption of adsorbed oxygen, which extends the resistance saturation time.Furthermore, NO 2 desorption caused by photogenerated holes delays the saturation of NO 2 adsorption.At the optimal point, the response time and recovery time showed 47 and 49 s.To ensure the effect of light activation, the device temperature was measured both before and after operation, as depicted in Fig. S16.The thermographic images demonstrate no significant temperature variation, indicating that the gas sensing performance is exclusively driven by light activation.
The outstanding response of the μLED gas sensor to NO 2 can be attributed to both the material characteristics of SnO 2 and the structural characteristics of the μLED-integrated gas sensor.The detailed material characteristics of SnO 2 NPs are explained in Note S2.In the case of the structural characteristics, we focused on the geometrical effects of the SnO 2 NPs on the electrode of the μLED gas sensor to clarify the volcano-like phenomenon in Fig. 2b. Figure 2c illustrates the schematics of the gas sensing reaction categorized into three phases with different light intensities.FDTD simulation was employed to predict the light absorbance of SnO 2 NPs at different light intensities, as shown in Fig. 2d.SnO 2 NPs were assumed to possess a spherical shape with densely packed positions.The simulated absorption distribution along the z-axis was obtained for light intensity of 50, 200, and 500 mW cm −2 , respectively.The first phase in the left panel of Fig. 2c indicates the low light intensity, where the lowest layers are exclusively activated and react with NO 2 .In this phase, all the generated photo-carriers react with the NO 2 molecules because the number of photocarriers is considerably smaller than the number of NO 2 molecules.As the light intensity increases, activation layer of SnO 2 NPs increases, leading to reactions with more NO 2 molecules.The response of the sensor reaches maximum value when the activation layer of SnO 2 NPs is equal to the thickness of sensor electrodes, as shown in the middle panel of Fig. 2c.However, when the activation layers of SnO 2 NPs become thicker than sensor electrodes due to excessive light intensity, response of the sensor diminishes, as shown in the right panel of Fig. 2c.When the NO 2 is introduced, most of the NO 2 molecules react with SnO 2 NPs located on the top layer.Due to the relatively weak gas reaction in the bottom layers, the resistance of the top layers increases, while the resistance of bottom layers shows little increase.Consequently, the majority of current flows through the bottom layer with low resistance, resulting in a decrease in the response of the sensor.Throughout these three phases, the dependency of the response curve on the light intensity exhibits a volcano-like plot.In conclusion, the material characteristics of SnO 2 and the structure characteristics of the sensor device generate a synergistic effect on gas sensing performance.
To evaluate the stability and reliability, the sensor was exposed to 16 repetitive pulses of NO 2 under blue light illumination with optimal intensity, as shown in Fig. 3a.The sensor demonstrated reliability with sustainable base resistance and response.Figure 3b illustrates the response curves of SnO 2 to NO 2 with concentrations ranging from 200 to 1000 ppb.The sensor showed reversible sensing with a high response even at a low gas concentration of 200 ppb.The linear-fitted curve presented in Fig. 3c yielded a slope of 2.24 ppb −1 with an R 2 value of 0.9949, supporting the linearity between gas response and NO 2 concentration.The theoretical detection limit (DL) was determined based on signal-to-noise ratio of 3, using the following equations [9]: where y i represents the values of the response curve before NO 2 exposure, and y denotes the average value of y i .R x 2 value is calculated by a fifth-order polynomial fitting that extracted 10 data points from y i .The root means square noise (rms noise ) is defined as Eq. ( 5), with N representing the number of extracted data points.Through these calculations, the DL was found to be 4.46 ppt under blue light (5) DL = 3 rms noise slope  6) [42,43].At low humidity levels, the conduction mechanism is proton transport by hopping due to the adsorbed OH − ions [44].At high humidity levels, the charge transport occurs via the Grotthuss chain reaction, as shown in Eq. ( 7) [45][46][47]: The response to 2 ppm of NO 2 increased from dry air to RH50% and decreased from RH50% to RH80%.At low humidity levels, pre-chemisorbed H + assists NO 2 adsorption on SnO 2 , which enhances the response to NO 2 [48].
In contrast, at high humidity levels, excessive H 2 O adsorbs to Sn active sites in SnO 2 and fully occupies them with hydroxyl ions (Sn-OH − ), which inhibits H + enhanced NO 2 adsorption, leading to a decrease in response to NO 2 .The long-term stability of the sensor to 5 ppm of NO 2 was investigated by comparing the initial state and after 6 months, as shown in Fig. S19.The sensor maintained a high response and short response time, despite a slight increase in baseline resistance and a decrease in response.The comparison of sensing performance to the state-ofthe-art light-activated gas sensors is depicted in Fig. 3d [14,[49][50][51][52][53][54], and detailed properties are summarized in Table S1.The left panel compares the response to power, while the right panel compares the response to response time.Our device exhibited a response time of 47 s to 5 ppm of NO 2 with 6928 of response, which is incomparable to other works.Moreover, the response was significantly high with microwatt-scale power consumption (63.2 μW).Furthermore, most reported works utilized UV-LEDs as a light source to activate metal oxide-based gas sensors.In contrast, our device demonstrated superior performance under blue light illumination.

Noble Metal Decoration on SnO 2 NPs
The noble metals (Au, Pd, and Pt) were decorated on SnO 2 NPs to investigate the tunable gas selectivity to reducing gases.For noble metal decoration, SnO 2 NPs were dissolved in DI water and sonicated to make a colloidal solution.After sonication, noble metal precursors (H 2 AuCl 4 , K 2 PdCl 4 , and K 2 PtCl 4 ) were put into SnO 2 NPs solution.Pluronic F-127 was additionally put into SnO 2 solution for surfactant and reducing agent [55].Pluronic F-127 is a block copolymer composed of polyethylene oxide (PEO) and polypropylene oxide (PPO) with a PEO-PPO-PEO triblock structure.Due to the hydrophilic PEO and hydrophobic PPO, Pluronic F-127 constructed a micelle structure in a water-based solution [56].SnO 2 NPs were surrounded by Pluronic F-127 and noble metal precursor can permeate into SnO 2 NPs [57].Furthermore, Pluronic F-127 facilitated the reduction of metal ions with good stabilization [58].Overall, through the solution process-based noble metal decoration, Au, Pd, and Pt were uniformly decorated onto the SnO 2 NPs.Diverse characterizations were conducted to confirm the morphology and chemical states of noble metal NPs. Figure 4a shows TEM images of Au-decorated SnO 2 NPs (Au-SnO 2 NPs), Pd-decorated SnO 2 (Pd-SnO 2 NPs), and Pt-decorated SnO 2 (Pt-SnO 2 NPs).The size of the SnO 2 NPs was measured to be around 10 nm, whereas Au and Pd were confirmed to be about 5 nm and Pt was measured to be 3 nm.High-resolution images were obtained using TEM to investigate the crystallinity of the Au-SnO 2 NPs, Pd-SnO 2 NPs, and Pt-SnO 2 NPs, as shown in Fig. S20.The crystal structure of each material was confirmed with diffraction patterns, which are presented in the inset of each image.Moreover, STEM with energy-dispersive XPS analysis was performed to observe the distribution of the noble metal NPs on the SnO 2 NPs.As shown in the EDS images in Fig. 4b, noble metal NPs were well-dispersed throughout each sample.XPS analysis was conducted to confirm the surface chemical states of the noble metal-decorated SnO 2 NPs. Figure S19 presents the XPS spectra of Sn 3d and O 1s of Au-SnO 2 NPs, Pd-SnO 2 NPs, and Pt-SnO 2 NPs.Additionally, O 1s peaks of noble metal-decorated SnO 2 NPs were deconvoluted into three peaks, and the peak location is summarized in Table S2. Figure 4c displays the XPS spectra of Au, Pd, and Pt, which were conducted to investigate the surface chemical bonding state of noble metal NPs.The detailed deconvoluted peak positions are provided in Table S3.The presence of oxidized peaks in the noble metal decorated on SnO 2 NPs revealed that the surface of the noble metals was partially oxidized, while the inner region remained in a metallic phase [59].

Reducing Gas Sensing Properties of Metal-Decorated SnO 2 NPs
The was defined using Eq. ( 8), which represents the response of reducing gases as the ratio of the resistance between air gas atmosphere.
Here, R g and R a are the same as Eq. ( 2).All sensors showed distinct selectivity patterns under blue light illumination.
As shown in Fig. S23  Based on the conventional mechanism of chemoresistive gas sensors, the reaction with reducing gases leads to a decrease in the resistance of n-type SnO 2 .However, the gas response curves of our gas sensor array showed different patterns compared to conventional gas sensors.These unique sensing patterns can be explained by the oxygen species on the surface of SnO 2 NPs.Oxygen molecules exist as ionized oxygen species on the surface of SnO 2 NPs, especially in the form of O 2 − at room temperature [60].When a reducing gas such as CH 3 COCH 3 is exposed to SnO 2 , the stable form of O 2 − is less reactive with CH 3 COCH 3 [61].Thus, for the reaction to occur, it is assumed that O 2 − dissociates into O − before reacting with CH 3 COCH 3 , which increases the resistance, as shown in Eq. ( 9).After that, the reaction of CH 3 COCH 3 with O − decreases the resistance, as shown in Eq. (10).
To supplement this mechanism, the dynamic response shape of SnO 2 NPs to CH 3 COCH 3 was measured under various conditions of temperature, light, and atmosphere, as shown in Fig. S26.When CH 3 COCH 3 was to SnO 2 NPs in the dark state at room temperature, the resistance increased because oxygen dissociation was more dominant than the decomposition of CH 3 COCH 3 .As the temperature increased, the resistance variation for the input of CH 3 COCH 3 showed opposite direction, resulting in a decrease of the resistance.The elevated temperature activates the adsorbed oxygen and decreases the activation energy for the reaction with CH 3 COCH 3 , making the decomposition of CH 3 COCH 3 more dominant than oxygen dissociation.At 150 °C, the tendency for resistance to decrease with the input of CH 3 COCH 3 was abruptly enhanced because O − becomes the stable oxygen species at 150 °C, and is reactive with reducing gases [60].In the case of light illumination on SnO 2 NPs, the resistance also increased with input of CH 3 COCH 3 at room temperature.However, the response and recovery were faster than in the dark state due to the light-activated oxygen species, which is more reactive than adsorbed oxygen, as shown in Eqs. ( 11) and ( 12) [4].
As the temperature increases with light illumination, the tendency of dynamic curve shape was similar to that in dark state.The elevated temperature causes the resistance to decrease with reaction of CH 3 COCH 3 .To clarify the effect of oxygen, CH 3 COCH 3 was measured under N 2 atmosphere.When CH 3 COCH 3 was exposed to SnO 2 NPs under N 2 atmosphere, the resistance decreased.CH 3 COCH 3 directly adsorbs SnO 2 NPs and draws electrons due to the absence of oxygen [62].Therefore, it can be assumed that the resistance increase in reducing gases at low temperature is due to oxygen dissociation of the stable oxygen species.Moreover, noble metal decoration on SnO 2 NPs results in distinct responses to various gases.In the case of SnO 2 NPs, C 2 H 5 OH showed a decrease in resistance, while other gases showed an increase in resistance.It is assumed that the decomposition of C 2 H 5 OH is more dominant than oxygen dissociation, unlike other reducing gases.When Au is decorated on SnO 2 , Au promotes oxygen dissociation with chemical sensitization, which decreases the activation energy for the reaction of reducing gases [63,64].The catalytic effect of Au increases the decomposition of reducing gases, which suppresses the increase in resistance caused by oxygen dissociation on SnO 2 .Therefore, the reducing gases showed a decrease in response, but C 2 H 5 OH showed an increase in response.This leads Au-SnO 2 NPs to exhibit good sensitivity and selectivity to C 2 H 5 OH.For Pd-SnO 2 NPs, Pd shows chemical sensitization to H 2 through phase transition of PdH x [65].The activation energy for the reaction between H 2 and oxygen decreases due to the dissolution of H 2 to H + ion.Therefore, H 2 reaction with oxygen is more dominant than oxygen dissociation, causing the resistance of Pd-SnO 2 NPs to H 2 to decrease.For Pt-SnO 2 NPs, Pt accelerates oxygen dissociation on SnO 2 , which causes the resistance of all reducing gases resistance to increase.This results in Pt-SnO 2 NPs showing high response for various reducing gases through an increase in resistance.Overall, the μLED platform was demonstrated to detect various reducing gases with tunable selectivity.

Real-Time Gas Monitoring with a Fully Hardware-Integrated E-Nose Array
Real-time gas monitoring of the μLED gas sensor array was tested to verify its suitability for daily use, as shown in Figs.6a and S27, and Video S1.The device was connected to a PCB via a Pt-wire-bonding process for operation, as shown in Fig. 6b.The MCU provided the 3.3 V operation voltage to the μLEDs, and the 0.5 V sensing voltage to each sensing component to monitor the responses simultaneously and independently.Figure 6c shows the customized design of PCB, which ensured the μLEDs deliver separate electrical signals in response to different target gases.The device is connected to a mobile phone via Wi-Fi for gas hazard notification.The MCU is programmed to send a signal when the resistance change induced by the gas reaction exceeds a predetermined threshold, as shown in Fig. S28 and Note S3.If the target gas concentration is over the threshold value, the signal is transmitted, and the corresponding response value appears in the mobile application.The application activates colorcoded alarms, displays warning text messages, and monitors the gas response from each sensor.Figure 6d shows the user interface of the mobile application when the sensor detects NO 2 gas.By utilizing these platforms, µLED gas sensor array can identify the distinct responses of each sensor and compare their selectivity via wireless communication.Moreover, the practical applications of µLED gas sensor array were demonstrated by detecting a fermented skate (Raja kenojei), hydrogen leakage, and wine, as shown in Fig. 6e and Video S2.Fermented skate is associated with NH 3 , which is produced by microbial activity through the degradation of urea and trimethylamine oxide [66].Detecting hydrogen leakage in gas pipes is crucial for the prevention of hydrogen explosion, which has the possibility of ignition above 4% of H 2 [67].Wine, as an alcoholic beverage, contains C 2 H 5 OH.When the different species were introduced to the gas sensor array, the sensor signals showed distinct responses, as shown in Fig. S29.
In conclusion, the real-time detection of distinct 4 gases and practical application to 3 different species by µLED gas sensor array demonstrated the potential capability for establishing advanced e-nose platforms.

Conclusion
The μLED-integrated SnO 2 NPs gas sensor array is capable of selectively detecting various gases at room temperature while operating at extremely low power levels.Fabrication of the 20 × 20 μm 2 sized μLED gas sensor platform was conducted via a conventional semiconductor device fabrication process.By integrating μLEDs with the gas sensor, blue light activation of sensing materials with micro-watt power consumption was achieved.The 1.5 μm gap between the light source and the sensing materials maximizes the light activation efficiency.The μLED-integrated SnO 2 NPs gas sensor exhibited a high response to 5 ppm of NO 2 (6928) with a power consumption of 63.2 μW.The volcano shape of gas response, depending on the light intensity, was interpreted using FDTD simulation of light absorbance.For selective detection of reducing gases, noble metals such as Au, Pd, and Pt were decorated onto SnO 2 NPs.With the catalytic effect of the noble metals, the noble metal-decorated SnO 2 NPs gas sensor array showed a distinct pattern in response to reducing gases.Real-time gas monitoring was achieved by connecting the gas sensor to a mobile phone via Wi-Fi using an MCU.Overall, this work is expected to contribute to the enlargement of μLED gas sensor fields and diversify the detectable gases for our healthy living environment.

Fig. 1
Fig. 1 µLED-integrated gas sensor array with SnO 2 NPs.Schematic illustration of a structure, and b process to gas detection of µLEDsintegrated gas sensor.c Optical image of µLEDs-integrated gas sensor array.d Optical images of µLEDs-integrated gas sensor array with different light intensities.e PL spectrum of µLEDs.f TEM image, g absorbance plot and Tauc plot, and h PL spectrum of SnO 2 NPs

Fig. 2
Fig. 2 NO 2 gas responses of SnO 2 NPs under different light intensities.a Dynamic gas sensing curves to 5 ppm of NO 2 under different light intensities.b Optimization of NO 2 response by injection current.c Schematic illustrations of gas sensing reaction under 3 different light intensities.d Volume absorption profile of model close-packed SnO 2 NPs by FDTD simulation.The cross-sectional area is set to 10 × 360 nm 2

Fig. 3
Fig. 3 Light-activated NO 2 sensing properties of SnO 2 NPs. a Response curves to 16 pulses of 5 ppm of NO 2 .b Response curves to 200-1000 ppb of NO 2 .c Calibration of response and detection limit plot.d Comparison of response time vs. response with the state-of-the-art lightactivated gas sensors for NO 2 detection

( 8 )
R r = R g − R a ∕R g ⋅ 100 (%) detailed in Fig. S24.The gas mixtures containing 50 ppm of NH 3 , H 2 , and C 2 H 5 OH were measured by SnO 2 NPs, Au-SnO 2 NPs, Pd-SnO 2 NPs, and Pt-SnO 2 NPs, which exhibited distinct patterns of dynamic curves, as shown in Fig. S25.Overall, noble metal-decorated SnO 2 NPs-based 2 × 2 gas sensor array demonstrated the ability to discriminate various gases under light illumination.

Fig. 6
Fig. 6 Real-time gas monitoring using a mobile application.a Schematic illustration of real-time gas sensing platform setup.b Optical image of MCU-integrated µLED gas sensor module.c A schematic illustration of PCB circuit design.d User interface of the mobile gas notification application when the module selectively detects NO 2 gas.e Response plots of the practical application of gas sensor array to detect fermented skate, hydrogen leakage, and wine

2 NPs, Au-SnO 2 NPs, Pd-SnO 2 NPs, and Pt-SnO 2 NPs
, the sensors were exposed to 4 repetitive pulses of reducing gases, exhibiting high reliability and repeatability.For a better comparison, the polar plots of SnO 2 NPs, Au-SnO 2 NPs, Pd-SnO 2 NPs, and Pt-SnO 2 NPs to reducing gases were obtained, as shown in Fig.5b.SnO 2 NPs exhibited moderate response to NH 3 , CO, and C 2 H 5 OH, Au-SnO 2 NPs exhibited high sensitivity and selectivity toward C 2 H 5 OH, Pd-SnO 2 NPs showed a good response to H 2 , and Pt-SnO 2 NPs exhibited a good response to several gases, especially NH 3 .The response tendency of reducing gases under various light intensities was measured and is